Comparison between the principle of linear power supply and switching power supply
Introduction to linear power supplies:
The linear power supply first transforms the AC power through a transformer, and then rectifies and filters the rectifier circuit to obtain an unsteady DC voltage. To achieve a high-precision DC voltage, the output voltage must be adjusted through voltage feedback. From the main performance point of view, this power supply technology is very mature, can achieve high stability, the ripples are very small, and there is no interference and noise that switching power supplies have. The voltage feedback circuit works in a linear state, and there is a certain voltage drop on the adjustment tube. When a large operating current is output, the power consumption of the adjustment tube is too large and the conversion efficiency is low.
Linear power supply means that the tube used for voltage adjustment works in the linear region. Correspondingly, switching power supply means that the tube used for voltage adjustment works in the saturation and cut-off areas, that is, the switching state.
Linear power supplies generally sample the output voltage and then send it to a comparison voltage amplifier with the reference voltage. The output of this voltage amplifier is used as the input of the voltage adjustment tube to control the adjustment tube so that its junction voltage changes with the change of the input, thereby adjusting its output. Voltage. But the switching power supply changes the output voltage by changing the on and off time of the regulating tube, that is, the duty cycle.
The tubes used for voltage regulation in linear power supplies work in the linear region. Correspondingly, switching power supply means that the tube used for voltage adjustment works in the saturation and cut-off areas, that is, the switching state.
Linear power supplies generally sample the output voltage and then send it to a comparison voltage amplifier with the reference voltage. The output of this voltage amplifier is used as the input of the voltage adjustment tube to control the adjustment tube so that its junction voltage changes with the change of the input, thereby adjusting its output. Voltage. But the switching power supply changes the output voltage by changing the on and off time of the regulating tube, that is, the duty cycle.
Principle of linear power supply: Linear power supply mainly includes power frequency transformer, output rectifier filter, control circuit, protection circuit, etc. The linear power supply first transforms the AC power through a transformer, and then rectifies and filters the rectifier circuit to obtain an unsteady DC voltage. To achieve a high-precision DC voltage, the output voltage must be adjusted through voltage feedback. This power supply technology is very mature and can achieve very high precision. High stability, very small ripples, and no interference and noise like switching power supplies. However, its disadvantage is that it requires a large and heavy transformer, and the required filter capacitor is also quite large in size and weight. Moreover, the voltage feedback circuit works in a linear state, and there is a certain voltage drop on the adjustment tube. When the large operating current is output, At this time, the power consumption of the adjustment tube is too large, the conversion efficiency is low, and a large heat sink must be installed. This kind of power supply is not suitable for the needs of computers and other equipment and will be gradually replaced by switching power supplies.
The switching power supply rectifies the alternating current into direct current, then inverts the direct current into alternating current, and then rectifies the output into the required direct current voltage. In this way, the switching power supply eliminates the need for the transformer and voltage feedback circuit in the linear power supply. The inverter circuit in the switching power supply is completely digitally adjusted and can also achieve very high adjustment accuracy.
The main working principle of the switching power supply is that the upper and lower bridge Mos tubes are turned on in turn. First, the current flows in through the upper bridge Mos tube. The storage function of the coil is used to gather the electric energy in the coil. Finally, the upper bridge Mos tube is closed and the lower bridge Mos tube is turned on. The bridge's Mos tube, coil and capacitor continue to supply external power. Then the lower bridge Mos tube is closed, and then the upper bridge is opened to let the current enter, and this is repeated. Because the Mos tube needs to be turned on and off in turn, it is called a switching power supply.
The linear power supply is different. Since there is no switch involved, the water supply pipe is always discharging water. If there is too much water, it will leak out. This is why we often see that the Mos tube of some linear power supplies generates a lot of heat. The endless electrical energy is all converted into heat energy. From this perspective, the conversion efficiency of the linear power supply is very low, and when the heat is high, the life of the components will inevitably decrease, affecting the final use effect.
The main difference between switching power supplies and linear power supplies is the way they work.
Linear power supply power devices work in a linear state, which means that the power device is always working as soon as it is used, so its working efficiency is low, usually between 50[[%]]~60[[%]], and It must be said that it is a very good linear power supply. The working method of linear power supply requires a voltage converter device to change from high voltage to low voltage. Generally, it is a transformer, but there are also others like KX power supply, which then rectifies and outputs DC voltage. As a result, it is very large, bulky, inefficient, and generates a lot of heat. It also has its advantages: small ripple, good adjustment rate, and small external interference. Suitable for use with analog circuits, various amplifiers, etc.
Switching power supply. Its power device works in a switching state (on and off, on and off, the frequency is very fast, the frequency of general flat-panel switching power supply is 100~200KHz, and the frequency of module power supply is 300~500KHz). In this way, its loss will be small and its efficiency will be high. There are also requirements for the transformer, which must be made of materials with high magnetic permeability. It's a little stained, but his transformer is as small as a character. The efficiency is 80% to 90%. It is said that the best VICOR modules in the United States are as high as 99%. The switching power supply has high efficiency and small size, but compared with the linear power supply, its ripple and voltage and current regulation rate are at a discount.
